Low power, power on reset circuit with accurate supply voltage detection

ABSTRACT

The power on reset circuit includes: a comparator; a resistor string having a first end coupled to a first supply node of the comparator, a first tap point node coupled to a first input of the comparator, and a second end coupled to a second input of the comparator; and a diode connected transistor device coupled between the second end of the resistor string and a second supply node of the comparator.

FIELD OF THE INVENTION

The present invention relates to electronic circuitry and, inparticular, to a low power, power on reset circuit with accurate supplyvoltage detection.

BACKGROUND OF THE INVENTION

Many prior art power on reset circuits exist of which a majority aremeant for digital applications for which the reset needs to release oncethe power supply has reached a level safe for the digital circuitry tostore voltages for a known startup condition. Thus the reset level caneffectively track the threshold voltages (VT's) of the MOS devices andcan vary with the large VT process variations (3:1 in some cases overtemp and process). If analog circuitry exists there may be a need for amore accurate reset so that reliable operation of the analog circuits isachieved. This is often a more complicated function than VT variationand may require an accurate band-gap based power on reset, which isstable over process and temperature variations.

FIGS. 1 and 2 show examples of prior art power on reset circuits.

The circuit of FIG. 1 can be designed for low power applications with acurrent consumption of about 10 nA with a total resistance of about 80Mohms. The circuit functions well for digital applications and has awell-known VT dependence for the power supply threshold. The trip pointcan vary from 0.8V down to 0.2V typically. If the circuit were used foranalog applications as simple as an internal clock oscillator, theoscillator could malfunction corrupting the on chip clock andsubsequently corrupting the digital data that the clock system drives.This is especially problematic if the circuit undergoes a sudden powerloss.

The circuit of FIG. 2 is an example of an accurate band-gap comparator.This solution combines the function of a band-gap reference with thesupply tap comparator. Resistors R2 and R3 are the supply samplingstring resistors and the supply voltage VDD is divided down byR3/(R3+R2). This divided down supply goes into the band-gap comparatorwhich switches state when the input (bases of transistors Q0 and Q1) areequal to the band gap voltage. Transistor Q1 is run at a lower currentdensity than transistor Q0, and resistor R0 completes the well knownproportional-to-absolute-temperature (PTAT) loop. Resistor R1 thus has aPTAT voltage across it (at the trip point) and transistor Q0'sbase-to-emitter voltage (vbe) completes the band gap voltage at thebases of transistors Q0 and Q1. The resistors must be sized properly fora given technology to make the comparator stable over process andtemperature.

One drawback to this design is the required voltage headroom foroperation is greater than the band-gap voltage (˜1.2V). The otherdrawback is the need for several large resistors since low currentconsumption is desired. The total resistance can total 100's of Mohmsfor current consumption of 10's of nA. This creates a significant diesize penalty.

SUMMARY OF THE INVENTION

A power on reset circuit includes: a comparator; a resistor stringhaving a first end coupled to a first supply node of the comparator, afirst tap point node coupled to a first input of the comparator, and asecond end coupled to a second input of the comparator; and a diodeconnected transistor device coupled between the second end of theresistor string and a second supply node of the comparator.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a circuit diagram of a prior art power on reset circuit;

FIG. 2. is a circuit diagram of a prior art power on reset circuit witha band gap comparator;

FIG. 3. is a circuit diagram of a band gap comparator power on resetcircuit, according to the present invention;

FIG. 4. is a circuit diagram of band gap comparator power on resetcircuit with the addition of well defined hysteresis, according to thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3 shows a simple implementation of a band gap comparator techniquethat minimizes the total resistance needed and can operate at 1.2V andlower (with some accuracy loss), according to the present invention. Thecircuit of FIG. 3 includes MOS transistors M6 and M7; bipolartransistors Q3, Q4, and Q5; resistors R6, R7, and R10; supply voltagesVDD and VSS; and output node OUT. Transistors M6, M7, Q3, and Q4 form aband gap comparator.

Transistor Q3 is designed to run at a lower current density at the trippoint than transistor Q4. Resistor R6 is the PTAT loop resistance andresistor R7 sets the current consumption. The comparator will trip whensupply voltage VDD is approximately 1.2V if the resistors are chosenproperly. If supply voltage VDD is 1.2V, the current in the resistorstring R6 and R7 is PTAT by definition since transistor Q5's vbe plusthe drop across resistors R6 and R7 equals the band-gap voltage. Iftransistor Q3 is eight times the emitter area of transistor Q4 then thewell known 54 mV difference (room temp) exists between the bases oftransistors Q3 and Q4 when the collector currents are equal (the trippoint). Resistors R7 and R6 are chosen such that this condition occurs.The comparator is stable over process and temp when designed for a tripvoltage of a band-gap voltage. The circuit can be designed for lowertrip voltages with more of a tolerance in variation over supply andtemp. The circuit can be designed for trip points at 800 mV plus orminus 10% over temperature and process variations, for example. ResistorR10 can be eliminated and is only used to reduce current consumption inthe comparator portion of the circuit. Resistor R10 is effective with10's of Kohms. The total resistance of 65 Mohms (resistor R7 plusresistor R6) can be used for 30 nA typical current consumption.

A big advantage of this circuit is the required resistance is minimizedfor a given current consumption. This is accomplished by droppingVDD-VBE across the resistor string R6 and R7 (instead of the full supplyvoltage VDD) and combining the supply voltage VDD sampling string withthe PTAT function of FIG. 2.

The circuit of FIG. 4 is similar to the circuit of FIG. 3 with theaddition of well defined hysteresis with the addition of a single PMOStransistor M10, and resistors R12, R13, and R14 instead of resistors R6and R7.

The present invention provides power on reset with 10's of nA currentconsumption that has predictable/stable supply voltage detectionthreshold. This band-gap comparator is optimized for low power andreduced die size.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

1. A power on reset circuit comprising: a comparator; a resistor stringhaving a first end coupled to a first supply node of the comparator, afirst tap point node coupled to a first input of the comparator, and asecond end coupled to a second input of the comparator; and a diodeconnected transistor device coupled between the second end of theresistor string and a second supply node of the comparator.
 2. Thecircuit of claim 1 wherein the comparator is a band gap comparator. 3.The circuit of claim 1 wherein the comparator comprises: a first MOStransistor coupled to the first supply node; a second MOS transistorcoupled to the first supply node and having a gate coupled to a gate ofthe first MOS transistor; a first bipolar transistor coupled between thefirst MOS transistor and the second supply node, and having a baseforming the first input of the comparator; and a second bipolartransistor coupled between the second MOS transistor and the secondsupply node, and having a base forming the second input of thecomparator.
 4. The circuit of claim 3 further comprising a resistorcoupled between the first bipolar transistor and the second supply node.5. The circuit of claim 1 wherein the resistor string comprises: a firstresistor coupled between the first supply node and the first input ofthe comparator; and a second resistor coupled between the first resistorand the second input of the comparator.
 6. The circuit of claim 3wherein the resistor string comprises: a first resistor coupled betweenthe first supply node and the base of the first bipolar transistor; anda second resistor coupled between the first resistor and the base of thesecond bipolar transistor.
 7. The circuit of claim 1 wherein the diodeconnected transistor device is a bipolar transistor.
 8. The circuit ofclaim 1 further comprising a transistor coupled between the first supplynode and a second tap point of the resistor string, and having a controlnode coupled to an output of the comparator.
 9. The circuit of claim 8wherein the resistor string comprises: a first resistor coupled inparallel with the transistor; a second resistor coupled between thefirst resistor and the first input of the comparator; and a thirdresistor coupled between the second resistor and the second input of thecomparator.
 10. The circuit of claim 8 wherein the transistor is a MOStransistor.
 11. The circuit of claim 3 further comprising a transistorcoupled between the first supply node and a second tap point of theresistor string, and having a control node coupled to an output of thecomparator.
 12. A power on reset circuit comprising: a comparator; afirst resistor coupled between a first supply node of the comparator anda first input of the comparator; a second resistor coupled between thefirst input of the comparator and a second input of the comparator; anda diode connected bipolar device coupled between the second resistor anda second supply node of the comparator.
 13. The circuit of claim 12wherein the comparator is a band gap comparator.
 14. The circuit ofclaim 12 wherein the comparator comprises: a first MOS transistorcoupled to the first supply node; a second MOS transistor coupled to thefirst supply node and having a gate coupled to a gate of the first MOStransistor; a first bipolar transistor coupled between the first MOStransistor and the second supply node, and having a base forming thefirst input of the comparator; and a second bipolar transistor coupledbetween the second MOS transistor and the second supply node, and havinga base forming the second input of the comparator.
 15. The circuit ofclaim 14 further comprising a third resistor coupled between the firstbipolar transistor and the second supply node.
 16. The circuit of claim12 further comprising: a third resistor coupled between the first supplynode and the first resistor; and a transistor coupled in parallel withthe third resistor, and having a control node coupled to an output ofthe comparator.
 17. The circuit of claim 16 wherein the transistor is aMOS transistor.